The present invention relates to optical filters and more particularly to such devices for photodetectors.
In many applications of photodetectors, filters are used to isolate spectral bands of light so that the devices are sensitive to only a narrow spectral bandwidth. Both interference and absorption filters are commonly used in these applications. Problems have often arisen in the application of flat filters of these types when the device which they are used is to have a fairly uniform response to light from wide incident angles, for example from 0.degree. to 45.degree.. The spectral transmission characteristic of filters varies depending upon the incident angle at which the light strikes the filter. In the case of an interference type of filter, increasing the angle of incidence shifts the spectral transmission characteristic to shorter wavelengths. In the case of absorption type filters, the attenuation at a given wavelength varies exponentially with the length of path through the filter. For a flat filter, the path length varies inversely with the cosine of the angle of refraction within the filter. The result can be a shift of the cut-off characteristic of the absorption filter. Shifting of the passband of spectral transmission is highly undesirable for devices which are intended to be responsive over a limited and specific spectral bandwidth.